The present disclosure relates generally to photolithographically patterned spring contacts, and more particularly to one or more released springs embedded in a laminate structure.
Photolithographically patterned spring devices (referred to herein as “microsprings”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. Such microsprings are disclosed and described, for example, in U.S. Pat. No. 5,914,218, which is incorporated by reference herein. A microspring is generally a micrometer-scale elongated metal structure having a free (cantilevered) portion which bends upward from an anchor portion which is affixed directly or indirectly to a substrate.
The microspring is formed from a stress-engineered metal film (i.e., a metal film fabricated to have a stress differential such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The microspring is attached to the substrate (or intermediate layer) at a proximal, anchor portion thereof. The microspring further includes a distal, tip portion which bends away from the substrate when the release material located under the tip portion is removed (e.g., by etching) or it is otherwise released.
The stress differential is produced in the spring material by one of several techniques. According to one technique, different materials are deposited in layers, each having a desired stress characteristic, for example a tensile layer formed over a compressive layer. According to another technique a single layer is provide with an intrinsic stress differential by altering the fabrication parameters as the layer is deposited. The spring material is typically a metal or metal alloy (e.g., Mo, MoCr, W, Ni, NiZr, Cu), and is typically chosen for its ability to retain large amounts of internal stress, electrical conductivity, and intrinsic strength. Microsprings are typically produced using known photolithography techniques to permit integration of the microsprings with other devices and interconnections formed on a common substrate. Indeed, such devices may be constructed on a substrate upon which electronic circuitry and/or elements have been or are formed.
The process of forming stress-engineered microsprings facilitates the formation of arrays of devices with contact points out of the plane in which the devices are initially formed. Linear and 2-d arrays can thereby be produced. In additional, multiple stress-engineered microsprings may be formed in an overlying relationship such that, for example, a lower stress-engineered microspring provides structural support for an upper stress-engineered microspring, as disclosed in U.S. Pat. No. 7,550,855, incorporated herein by reference.
Microsprings find a wide range of applications, such as probe cards, electrical bonding to integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, scanning probes, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion of a microspring is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the microspring as an electrical contact).
Typically, the array of microsprings is formed with a tip-to-tip spacing selected according to the application of the array. For example, for probe testing, the tip-to-tip spacing would match the spacing of contact pads, leads or the like on the device under test. FIG. 18 is a microphotograph of an array 100 of microsprings released from a substrate.
Microsprings typically terminate at a tip, whose shape may be controlled photolithographically as the microspring is pattered in-plane. In certain applications, the microspring has a tip profile (e.g., an apical point) capable of physically penetrating an oxide layer that may form on the surface to which electrical contact is to be made. An example of a microspring 102 having an anchor portion 104 and a tip 106 shaped to pierce a layer such as an oxide illustrated in plan view in FIG. 19, although many different tip shapes may similarly be employed.
In order to provide a reliable contact with a surface to be contacted, the microspring must provide a relatively high contact force (the force which the spring applies in resisting a force oppositely applied from the surface to be contacted). This is particularly true in applications in which the apical point must penetrate an oxide layer. For example, some probing and packaging applications require a contact force on the order of 50-100 mg between the tip and the structure being contacted.
One problem presented by common microspring structures and their applications is that once released, the springs are susceptible to breaking, either at a point along their length or at their point of connection (anchor) to the substrate. Microsprings can be broken by vertical compression beyond their elastic limit. Thinner springs are known to accommodate greater vertical deflection, but present a lower spring coefficient, meaning that for some applications they cannot provide sufficient counterforce, for example to penetrate an oxide layer. Thicker microsprings withstand greater applied force, but are known to be more brittle than thinner springs, and susceptible to bending beyond their elastic limit. One approach to reducing the risk of breaking, at least for thick microsprings, would be to form gap stops under a portion of the released part of the microspring. However, forming such stops would require extra lithographic steps, additional material, and still are only of limited effect in preventing breakage.
Microsprings springs can also break due to lateral forces applied at their sides, which twist the spring and often result in breaking near the anchor. Such forces may be applied during manufacture and handling of the microspring structure, placing the microspring structure into jigs, chucks or other holders for additional processing, alignment, assembly, etc. This results in a requirement that special handling precautions and techniques be employed when manipulating microspring arrays. For example, a wafer on which a microspring array has been formed cannot be held with normal contact vacuum wands without a significant risk of breaking one or more of the microsprings. Likewise, a wafer on which a microspring array is formed cannot be retain inverted on chucks, as is traditionally done and necessary for backside polishing of the wafer.
Photoresist is currently used to temporarily coat microsprings and protect them during dicing. It is possible to simply leave the photoresist, or a least a portion thereof, on the microsprings following release in order to provide additional structural strength to the individual springs. However, this requires expensive wet processing, processing of released microsprings, and other factors which add to cost and the risk of damaging the microsprings. In addition, the photoresist is not compatible with higher temperatures associated with solder processing.